The invention relates to an apparatus for substrate treatment by means of laser radiation comprising a rotating polygonal mirror by which at least one incident laser beam is reflected and is pivotable across an arrangement of converging lenses which lie next to one another and which are arranged at a spacing from the substrate corresponding to their focal lengths.
Such an apparatus is in particular used for the working of, for example, band-shaped materials by means of laser radiation, with the laser light being split into a plurality of quasi-simultaneous working positions and short pulses of very high frequency simultaneously being produced from the continuous light of the laser. The high pulse repetition rate results in correspondingly high working speeds.
A beam splitting by so-called beam splitters would be disadvantageous in that the resulting part beams would have a correspondingly lower intensity which is not sufficient for specific working processes. The part beams would moreover also have a changing intensity, i.e. in any case a different intensity and/or spatial intensity distribution. The main problem with beam splitters is thus frequently the different influencing of the beam profile, in particular in the event of contamination (beam geometry).
When perforating paper by means of laser light beams, for example, it is absolutely necessary for the achieving of a uniform hole size and quality to ensure a specific, relatively high and constant intensity of the laser light beam carrying out the perforation. A beam splitting would thus again also be unfavorable here.
Apparatuses of the initially named kind (cf. e.g. DE-C-2918283) are in particular used for the perforation of thin papers such as cigarette tipping paper. The apparatus in each case produces a track of small holes per working head in this process, with the continuous light of the laser being chopped into individual pulsed beams with the help of an optical multiplexer. Every focused laser pulse evaporates the thin material instantaneously and thus produces a hole with a customary diameter of, for example, approximately 60 to approximately 150 μm. A respective apparatus can e.g. have 16 working heads and can thus perforate paper with, for example, four perforation zones each having four rows of holes and a width complementary thereto of approximately 150 mm at a web speed of several 100 m/min. The perforation is accordingly concentrated on a few zones transversely to the web and is arranged very densely in the longitudinal direction.
In practice, it is continually necessary to change the number of facets. For this purpose, up to now, the balanced polygon completely assembled with the drive motor has had to be replaced and the new polygon has to be adjusted again. This process, which takes several hours, is irritating for the end customer since only experienced and trained personnel can carry out the respective error-prone work. In practice, this has the result that a replacement of the polygon is only carried out reluctantly and very rarely.
However, the sweep angle or fan angle of the reflected laser beam and thus the number of the exposed and thus perforating individual beams can be changed by a respective polygon replacement and a correspondingly changed number of facets. As a rule, this is a question of the optimization of production speed. If, for example, only eight beams are required, i.e. for example eight perforation tracks, and if the respective machine is fitted with a polygon which, for example, exposes all sixteen individual paths present, eight of the sixteen beam paths at once have to be blocked if there is no conversion to a correspondingly different polygon. Fifty percent of the original laser energy is thus not available for the working. To block or suppress individual beams, the previously customary perforation machines of the corresponding type are provided with so-called single-beam shutters, i.e. for example with sixteen such shutters.
If the production quantity to be produced by the end user is very high and if the production time available for the respective production product amounts, for example, to several weeks, in practice, the time-consuming conversion to a polygon will previously have been selected which fans the laser light only to the e.g. eight beams required in order to utilize the available laser energy.